Received-SPF: pass (google.com: domain of linux-kernel+bounces-70157-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:40f1:3f00::1 as permitted sender) client-ip=2604:1380:40f1:3f00::1;
Subject: Re: [PATCH 2/5] mm: correct calculation of cgroup wb's bg_thresh in
 wb_over_bg_thresh
To: Tejun Heo <tj@kernel.org>
Cc: willy@infradead.org, akpm@linux-foundation.org,
 hcochran@kernelspring.com, mszeredi@redhat.com, axboe@kernel.dk,
 linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
References: <20240123183332.876854-1-shikemeng@huaweicloud.com>
 <20240123183332.876854-3-shikemeng@huaweicloud.com>
 <ZbAk8HfnzHoSSFWC@slm.duckdns.org>
 <a747dc7d-f24a-08bd-d969-d3fb35e151b7@huaweicloud.com>
 <ZbgR5-yOn7f5MtcD@slm.duckdns.org>
 <ad794d74-5f58-2fed-5a04-2c50c8594723@huaweicloud.com>
 <ZcUsOb_fyvYr-zZ-@slm.duckdns.org>
From: Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <6505a0c5-5e22-3b25-65f5-2b44e885785d@huaweicloud.com>
Date: Sun, 18 Feb 2024 10:35:41 +0800
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101
 Thunderbird/60.5.0
Precedence: bulk
MIME-Version: 1.0
In-Reply-To: <ZcUsOb_fyvYr-zZ-@slm.duckdns.org>
Content-Type: text/plain; charset=gbk
Content-Transfer-Encoding: 7bit


on 2/9/2024 3:32 AM, Tejun Heo wrote:
> Hello, Kemeng.
> 
> On Thu, Feb 08, 2024 at 05:26:10PM +0800, Kemeng Shi wrote:
>> Hi Tejun, sorry for the delay as I found there is a issue that keep triggering
>> writeback even the dirty page is under dirty background threshold. The issue
>> make it difficult to observe the expected improvment from this patch. I try to
>> fix it in [1] and test this patch based on the fix patches.
>> Run test as following:
> 
> Ah, that looks promising and thanks a lot for looking into this. It's great
> to have someone actually poring over the code and behavior. Understanding
> the wb and cgroup wb behaviors have always been challenging because the only
> thing we have is the tracepoints and it's really tedious and difficult to
> build an overall understanding from the trace outputs. Can I persuade you
> into writing a drgn monitoring script similar to e.g.
> tools/workqueues/wq_monitor.py? I think there's a pretty good chance the
> visibility can be improved substantially.
Hi Tejun, sorry for the late reply as I was on vacation these days.
I agree that visibility is poor so I have to add some printks to debug.
Actually, I have added per wb stats to improve the visibility as we
only have per bdi stats (/sys/kernel/debug/bdi/xxx:x/stats) now. And I
plan to submit it in a new series.
I'd like to add a script to improve visibility more but I can't ensure
the time to do it. I would submit the monitoring script with the per wb
stats if the time problem does not bother you.
Thanks.
> 
> Thanks.
>